Thixotropy is reversible time-dependent shear thinning of liquids and semi-solids. There are a number of advantages in using thixotropic dispersions in the colouring of materials such as plastics, inks and surface coatings. For example, these dispersions are easier to handle than a Newtonian liquid, because they are less likely to drip or spill. If used directly as a printing ink, for instance, they are less likely to run (flow down or spread laterally) compared with a Newtonian liquid. They resist settling of the pigment during storage. They are easier to dispense than a stiff solid. Finally, when the dispersions are first prepared, entrapped air-bubbles (if present) may be readily removed by vibrating or vertically tapping the sample.
As used in this specification, the term “pigment” refers to colorants, which are either water soluble pigments or water insoluble pigments being substances such as lake dyes, titanium dioxide and carbon black. Typically, such pigments are provided as fine, dry powders which are then incorporated into various products in order to impart colour either to the product itself or in the case of surface coatings or inks, to the surface to which they are applied or printed.
Owing to the fact that pigments are often difficult to disperse in the product to be coloured, it is usual that the pigment is formulated into a premix that may be more readily dispensed and included in the product. This ensures that the product achieves a uniform colour by virtue of the ease with which the premix is incorporated and evenly mixed into the product.
In order to ensure that the greatest depth of colour is achieved in a finished product, it is desirable that the concentration of pigment in a premix is maximised. This also has the added advantage that the level of additives in the premix, other then pigment, is minimised.
However, water insoluble pigments are notoriously difficult to disperse and provide as stable premixes. Efficient and effective pigment dispersion is necessary in order to obtain optimum and consistent colour strength. An effective dispersion process consists of the permanent breaking down of agglomerates into primary particles that are uniformly dispersed in the premix. In general terms, key steps in a dispersion process are:
Deagglomeration in which agglomerates and aggregates are broken down by the shear forces of the equipment being used to form the dispersion. This is followed by wetting out, which occurs at the surface of a pigment when a liquid and/or surfactant adheres to the pigment's surface and acts to displace air and moisture from the surface. The pigment must then be equally dispersed throughout premix. Generally, a lower viscosity tends to lead to a more even pigment distribution. Finally, the premix must be stabilised to prevent the pigment from re-agglomerating. Such stabilisation usually requires that the premix have a higher viscosity.
However, in order to achieve the most effective and efficient incorporation of the premix into the product to be coloured, it is desirable that the viscosity of the premix is minimised. Accordingly it will be evident that there is a conflict between on the one hand the stability of the premix and on the other hand, the efficiency of its incorporation into a product.
The present inventor has recognised this conflict in relation to water insoluble pigments and in the invention described below, seeks to provide a premix composition that is thixotropic in character. Such a composition has the advantage of being relatively high in viscosity when not sheared and hence conferring relative stability. However, once sheared by mixed, the viscosity is sufficiently reduced so that the premix may be effectively and efficiently incorporated into a product to be coloured.
The present inventor has also recognised that by substituting water soluble pigments for water insoluble pigments, it is possible to attain Newtonian rheology in a premix which is desirable in some applications.